Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode.
In proton exchange membrane (“PEM”) type fuel cells, hydrogen is supplied to the anode as fuel, and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Typically, the ion conductive polymer membrane includes a perfluorosulfonic acid (“PFSA”) ionomer.
The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of electrically conductive elements or plates referred to as flow fields. The flow fields function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cells in stacks in order to provide high levels of electrical power.
In the current flow field designs, tunnels intersect with a metal bead for the coolant and the reactants to pass through. Typically, the tunnels/channels have a nearly square trapezoidal cross-section (i.e., wall angle greater than 45 degrees). The flow tunnels/channels cross section are designed by primarily considering the coolant/reactant flow. However, the existence of tunnels causes large contact pressure variation at the intersection between the metal bead and the tunnel.
The primary function of metal bead is to seal the coolant or reactants from leaking outside from headers. The sealing function is provided by the contact pressure on the metal bead. Ideally, uniform contact pressure is the most favorable case. Large contact pressure variation may cause extreme low pressure at certain spot which may cause leaking.
Accordingly, the present invention provides improved contact pressure having less pressure variation at the intersection between metal bead seals and the flow channels.